Nonwoven fabrics or materials comprise loosely assembled webs or masses of fibers bound together with an adhesive binder. Adequately bonded nonwoven fabrics have a variety of uses including the preparation of nonwoven innerliners for baby diapers and other products. It is known to form bonded nonwoven fabrics by impregnating, printing or otherwise depositing an adhesive bonding composition on a base web predominantly comprising relatively long fibers, including those of textile length of from about one-half inch (1.27 cm) to about 2 and one-half inches (6.35 cm), or more. These fibers may be of cellulosic or polymeric materials such as polyesters, polyamides, polyacrylates and the like. The base web of nonwoven fibers, to which the binder composition is applied, can be produced by carding, garnetting, air laying, paper making procedures, or other known operations.
With respect to disposable baby diapers and related products, a large market for nonwoven components of these products has developed because of their improved performance, comfort and convenience. Regarding comfort, an important function of diaper construction is keeping moisture away from an infant's skin while at the same time handling a full volume discharge of urine.
A disposable diaper typically comprises a fibrous innerliner which contacts the infant's skin, a layer of highly porous, loosely compacted cellulosic wadding, a paper-like, densified, highly compacted cellulosic fibrous layer integral with the loosely compacted wadding and an impervious backing sheet adhered to the densified layer throughout their interface. The innerliner normally is comprised of fibers, especially polyester fibers, that are held together by a binder. The innerliner is of porous construction and its fibers have less wettability for water than the fibers of the loosely compacted wadding, resulting in a tendency for liquid to flow from the facing web into the wadding.
Two important characteristics of nonwoven innerliners are strikethrough and rewet. Strikethrough is the ability of the innerliner to pass moisture into the wadding upon initial contact. Rewet is the tendency of moisture to move back through the innerliner after initial wetting. Low rewet, or hydrophobic rewet, is needed to keep moisture away from the infant's skin.
It is customary that the binder for the nonwoven innerliner also contains a wetting agent (strikethrough surfactant) which reduces the water repellency of the innerliner so that urine may readily pass through it and into the loosely compacted wadding. The passage of the urine through the innerliner extracts most of the water soluble wetting agents from the binder rendering the innerliner substantially impervious to rewetting.
Thus, the innerliner separating the cellulosic wadding from the infant must be strong both wet and dry, i.e., high wet and dry tensile strength, possess good strikethrough and demonstrate low rewet properties.
Current emulsion binders used to bond the nonwoven polyester fibers of diaper innerliners are typically deficient in one or more properties. For example, N-methylolacrylamide based emulsions produce high residual levels of formaldehyde in the final innerliner product. Acrylate based emulsions typically show blocking. Styrene-butadiene based emulsions often show yellowing, insufficient softness, odor and running problems. Vinyl acetate-ethylene based emulsions without an added crosslinker such as N-methylolacrylamide typically have poor wet tensiles and poor rewet.
U.S. Pat. No. 3,922,462 discloses a permanently absorbent nonwoven fabric comprising a web of fibers, a crosslinkable latex binder and a surfactant consisting of at least one bis-alkyl sulfosuccinate having alkyl substituents containing 13-14 carbon atoms.
The incorporation of acrylic acid and other carboxylic acid containing monomers into interpolymers is well known. Crosslinking with metal ions including aluminum and zirconium has previously been disclosed as being useful for the insolubilization of carboxylic acid group containing materials such as polyacrylic acid and starches containing carboxylic acid groups. U.S. Pat. Nos. 2,758,102 and 3,137,588 are illustrative.
In addition, the use of metal ions in resin compositions which can be deposited on other materials in a controlled manner are disclosed in U.S. Pat. Nos. 3,536,518; 3,649,330 and 4,084,033.
U.S. Pat. No. 3,536,518 discloses an aqueous resin binder dispersion which is stable at a pH of from about 7 to about 9. The binder dispersion is dispersed and stabilized by an anionic surfactant which is active at a pH from about 7 to about 9 but which is inactivated by the presence of free divalent or trivalent metal cations which are released at pH less than about 7 by a metal chelate having a divalent or trivalent cation. By substantially simultaneously applying the binder dispersion to a fibrous material and lowering its pH to less than 7, the binder material is precipitated onto the fibrous material. The anionic surfactant is selected from the class consisting of alkyl aromatic sulfonic acids, alkane sulfonic acids and carboxylic acids.
U.S. Pat. No. 3,649,330 discloses a composition of an emulsion polymerized resin, which has a pH of less than 7, and certain metal salts, wherein the metal ion has a valence of at least 3, and a method of controlling resin deposition on materials by pretreating the materials whereby they have an alkaline condition and by applying the composition to the material under alkaline conditions.
U.S. Pat. No. 4,084,033 discloses a method of depositing synthetic resins from their colloidal aqueous dispersions onto wet fibrous webs to form bonded fibrous nonwoven fabrics comprising the use of (1) metal complex coordination compounds and (2) synthetic resins and/or surfactants, at least one of which contains a specific coordinating ligand capable of being affected by ions of said metals to control the total migration of the resin binder during such deposition. The surfactants are those anionic surfactants disclosed in U.S. Pat. No. 3,536,518.
The use of phosphate ester surfactants in emulsion polymerization is also known. Representative of such art are U.S. Pat. No. 3,642,740; U.S. Pat. No. 4,290,931 and Ger. Offen. No. 2,533,043.
There is no teaching in this prior art with regard to binder compositions which afford nonwoven products having good tensiles and good low rewet characteristics.